As natural products such as building lumber have become more scarce and thus expensive, manufactured building products have been developed. Primed lap sidings are a good example of this and are widely used today in the building industry.
Shortly after primed lap sidings were introduced to the market, problems developed in the lower boards rotting adjacent roof areas where flashing was placed behind the siding and under the shingles in the normal manner.
As a result of this, manufacturers now require that the bottom of the sidings stop one inch above the roof shingle to prevent moisture from absorbing into the bottom edge thereof.
As a result of the above, it is a widespread practice for the carpenters to use a one by four inch board as a guide. One by fours, however, are in reality only three quarters of an inch thick and if rot occurs, this could void the manufacturers' warranty on the siding. Other carpenters guess at the one-inch spacing, quite often resulting in uneven cuts, particularly on dormers and the like.